Single-knit jersey fabrics are broadly used to make underwear and top-weight garments, such as T-shirts. Compared to woven structures, the knit fabric can more easily deform, or stretch, by compressing or elongating the individual knit stitches (comprised of interconnected loops) that form the knit fabric. This ability to stretch by stitch rearrangement adds to the wearing comfort of garments made from knit fabrics. Even when knit fabrics are constructed of 100% hard yarns, such as cotton, polyester, nylon, acrylics or wool, for example, there is some recovery of the knit stitches to original dimensions after imposed forces are removed. However, this recovery by knit stitch rearrangement generally is not complete because hard yarns, which are not elastomeric, do not provide a recovery force to rearrange the knit stitches. As a consequence, single-knit fabrics may experience permanent deformations or ‘bagging’ in certain garment areas, such as at the elbows of shirt sleeves, where more stretching occurs.
To improve the recovery performance of circular, single-knit fabrics, it is now common to co-knit a small amount of spandex fiber with the companion hard yarn. As used herein, “spandex” means a manufactured fiber in which the fiber-forming substance is a long-chain synthetic polymer comprised of at least 85% of a segmented polyurethane. The polyurethane is prepared from a polyether glycol, a mixture of diisocyanates, and a chain extender and then melt-spun, dry-spun or wet-spun to form the spandex fiber.
For jersey knit constructions in circular knit machines, the process of co-knitting spandex is called “plating.” With plating, the hard yarn and the bare spandex yarn are knitted parallel, side-by-side relation, with the spandex yarn always kept on one side of the hard yarn, and hence on one side of the knitted fabric. FIG. 1 is a schematic illustration of plated knit stitches 10 wherein the knitted yarn comprises spandex 12 and a multi-filament hard yarn 14. When spandex is plated with hard yarn to form a knit fabric, additional processing costs are incurred beyond the added cost of the spandex fiber. For example, fabric stretching and heat setting usually are required in the finishing steps when making elastic knit jersey fabrics.
By “circular knitting” is meant a form of weft knitting in which the knitting needles are organized into a circular knitting bed. Generally, a cylinder rotates and interacts with a cam to move the needles reciprocally for knitting action. The yarns to be knitted are fed from packages to a carrier plate that directs the yarn strands to the needles. The circular knit fabric emerges from the knitting needles in a tubular form through the center of the cylinder.
The steps for making elastic circular-knit fabrics according to one known process 40 are outlined in FIG. 4. Although process variations exist for different fabric knit constructions and fabric end uses, the steps shown in FIG. 4 are representative for making jersey knit elastic fabrics with spun hard yarns, such as cotton. The fabric is first circular knit 42 at conditions of high spandex draft and feed tensions. For example, for single-knit jersey fabrics made with bare spandex plated in every knit course, the prior-art feed tension range is 2 to 4 cN for 22 dtex spandex; 3 to 5 cN for 33 dtex; and 4 to 6 cN for 44 dtex (DuPont Technical Bulletin L410). The fabric is knit in the form of a tube, which is collected under the knitting machine either on a rotating mandrel as a flattened tube, or in a box after it is loosely folded back and forth.
In open-width finishing, the knitted tube is then slit open 44 and laid flat. The open fabric is subsequently relaxed 46, either by subjecting it to steam, or by wetting it by dipping and squeezing (padding). The relaxed fabric is then applied to a tenter frame and heated (for heat setting 46) in an oven. The tenter frame holds the fabric on the edges by pins, and stretches it in both the length and width directions in order to return the fabric to desired dimensions and basis weight. This heat setting is accomplished before subsequent wet processing steps and, consequently, heat setting is often referred to as “pre-setting” in the trade. At the oven exit, the flat fabric is released from the stretcher and then tacked 48 (sewed) back into a tubular shape. The fabric then is processed in tubular form through wet processes 50 of cleaning (scouring) and optional bleaching/dyeing, e.g., by soft-flow jet equipment, and then dewatered 52, e.g., by squeeze rolls or in a centrifuge. The fabric is then “de-tacked” 54 by removing the sewing thread and re-opening the fabric into a flat sheet. The flat, still wet, fabric is then dried 56 in a tenter-frame oven under conditions of fabric overfeed (opposite of stretching) so that the fabric is under no tension in the length (machine) direction while being dried at temperatures below heat-setting temperatures. The fabric is slightly tensioned in the width direction in order to flatten any potential wrinkling. An optional fabric finish, such as a softener, may be applied just prior to the drying operation 56. In some cases a fabric finish is applied after the fabric is first dried by a belt or tenter-frame oven, so that the finish is taken up uniformly by fibers that are equally dry. This extra step involves re-wetting the dried fabric with a finish, and then drying the fabric again in a tenter-frame oven.
Heat setting “sets” spandex in an elongated form. This is also known as redeniering, wherein a spandex of higher denier is drafted, or stretched, to a lower denier, and then heated to a sufficiently high temperature, for a sufficient time, to stabilize the spandex at the lower denier. Heat setting therefore means that the spandex permanently changes at a molecular level so that recovery tension in the stretched spandex is mostly relieved and the spandex becomes stable at a new and lower denier. Heat setting temperatures for spandex are generally in the range of 175 to 200° C. For the prior art process 40 shown in FIG. 4, the heat setting 46 commonly is for about 45 seconds or more at about 190° C.
If heat-setting is not used to “set” the spandex, after the fabric is knitted and released from the constraints of the circular knitting machine, the stretched spandex in the fabric will retract to compress the fabric stitches so that the fabric is reduced in dimensions compared to what those dimensions would be if the spandex were not present. Compression of the stitches in the knitted fabric has three major effects that are directly related to elastic knit fabric properties, and thereby usually renders the fabric inappropriate for subsequent cut and sew operations.
First, stitch compression reduces fabric dimensions and increases fabric basis weight (g/m2) beyond desired ranges for single jersey knit fabrics for use in garments. As a result, the traditional finishing process for elastic circular-knit fabric includes a fabric stretching and heating step, at sufficiently high temperatures and sufficiently long residence time, so that the spandex yarn in the knit will “set” at desired stretched dimensions. After heat setting, the spandex yarn will either not retract, or will retract only modestly below its heat-set dimension. Thus, the heat-set spandex yarn will not significantly compress the knit stitches from the heat-set dimensions. Stretching and heat setting parameters are chosen to yield the desired fabric basis weight and elongation, within relatively tight limits. For a typical cotton-jersey elastic single-knit, the desired elongation is at least 60%, and the basis weight ranges from about 140 to about 240 g/m2.
Second, the more severe the stitch compression, the more the fabric will elongate on a percentage basis, thus far exceeding minimum standards and practical needs. When a plated knit with elastic yarn is compared with a fabric knit without elastic yarn, it is common for the plated elastic knit fabric to be 50% shorter (more compressed) than the fabric without elastic yarn. The plated knit is able to stretch in length 150% or more from this compressed state, and such excessive elongation is generally undesirable in jersey knits for cut and sew applications. This length is in the warp direction of the fabric. Fabrics with high elongation in length (stretch) are more likely to be cut irregularly, and are also more likely to shrink excessively upon washing. Similarly, stitches are compressed by spandex in the width direction, so that fabric width is reduced about 50% as well, far beyond the 15 to 20% as-knit width reduction normally encountered with rigid (non-elastic) fabrics.
Third, the compressed stitches in the finished fabric are at an equilibrium condition between spandex recovery forces and resistance to stitch compression by the companion hard yarn. Washing and drying of the fabric can reduce the hard-yarn resistance, probably in part because of agitation of the fabric. Thus, washing and drying may permit the spandex recovery forces to further compress the knit stitches, which can result in unacceptable levels of fabric shrinkage. Heat-setting the knit fabric serves to relax the spandex and reduce the spandex recovery force. The heat setting operation therefore improves the stability of the fabric, and reduces the amount that the fabric will shrink after repeated washings.
Heat setting is not used for all varieties of weft knit elastic fabrics. In some cases a heavy knit will be desired, such as in double knits/ribs and flat sweater knits. In these cases, some stitch compression by the spandex is acceptable. In other cases, the bare spandex fiber is covered with natural or synthetic fibers in a core-spinning or spindle-covering operation, so that the recovery of the spandex and resultant stitch compression is restrained by the covering. In still other cases, bare or covered spandex is plated only on every second or third knit course, thereby limiting the total recovery forces that compress the knit stitches. In seamless knitting, a process wherein tubular knits are shaped for direct use while being knitted on special machines, the fabric is not heat set because dense, stretchy fabrics are intended. For circular-knit jersey elastic fabrics made for cutting and sewing, however, wherein bare spandex is plated in every course, heat setting is almost always required.
Heat setting has disadvantages. Heat setting is an extra cost to finish knit elastic fabrics that contain spandex, versus fabrics that are not elastic (rigid fabrics). Moreover, high spandex heat-setting temperatures can adversely affect sensitive companion hard yarns, e.g., yellowing of cotton, thereby requiring more aggressive subsequent finishing operations, such as bleaching. Aggressive bleaching can negatively affect fabric tactile properties, such as “hand,” and usually requires the manufacturer to include fabric softener to counteract bleaching. Also, heat-sensitive hard yarns, such as those from polyacryonitrile, wool and acetate, cannot be used in high-temperature spandex heat-setting steps, because the high heat-setting temperatures will adversely affect such heat-sensitive yarns.
The disadvantages of heat setting have long been recognized, and, as a result, spandex compositions that heat-set at somewhat lower temperatures have been identified (U.S. Pat. Nos. 5,948,875 and 6,472,494 B2). For example, the spandex defined in U.S. Pat. No. 6,472,494 B2 has a heat set efficiency greater than or equal to 85% at approximately 175–190° C. The heat set efficiency value of 85% is considered a minimum value for effective heat setting. It is measured by laboratory tests comparing the length of stretched spandex before and after heat setting to the before-stretched spandex length. While such lower heat setting spandex compositions provide an improvement, heat setting is still required, and the costs associated with it have not been significantly reduced.
The traditional practice of making and heat setting circular-knit fabrics has further disadvantages. The knit fabric emerges from a circular knitting machine in the form of a continuous tube. As the tube is formed in knitting, it is either rolled under tension onto a mandrel, or it is collected as a flat tube under the knitting machine by plaiting, or loose folding. In either case, the fabric establishes two permanent creases where the fabric tube has been folded or flattened. Although the fabric is “opened” by slitting the fabric tube along one of the creases, subsequent use and cutting of the fabric usually must avoid the remaining crease. This reduces the fabric yield (or the amount of knit fabric that can be further processed into garments).
New methods are sought for making circular-knit, elastic, single-knit jersey fabrics that have bare spandex plated in every knit course, and that avoid the costs and disadvantages associated with heat setting.